There is no knowing yet when Prince Harry and Meghan Markle will return to the U.K. after their appearance at the Platinum Jubilee celebrations last week. But according to a royal expert, the duke will likely reunite with Prince William for Princess Diana's memorial.

The brothers are expected to be there to celebrate the 25th death anniversary of their mother, who died in a car crash on Aug. 31, 1997. According to Richard Fitzwilliams, the reunion could likely prove "difficult" not just for the siblings, but also for those who know the late Princess of Wales.

"The moment Diana is in the news, Prince Charles and Camilla suffer," the royal expert told Entertainment Daily.

He added, "And also, with the 25th anniversary, we don't know what's going to happen. Quite a bit may, again, featuring William and Harry possibly. So I think that will be difficult."

Prince Harry and Meghan Markle were recently in the U.K. to celebrate the Platinum Jubilee with the royals. But it is believed that they did not get to spend time with Prince William and Kate Middleton because of their busy schedules.

The couple reportedly invited the Cambridges to Lilibet's birthday party at Frogmore Cottage on June 4. Unfortunately, they were miles away in Cardiff, Wales for a pre-scheduled engagement.

It is understood that palace aides made certain that the Duke of Sussex does not get pictured with his brother at any time during the celebrations. This is to avoid possible royal drama which could steal the spotlight from Queen Elizabeth II.

According to royal author Victoria Arbiter, the brothers did not get to spend time together to patch things up. At some point in the future, they would have to sit down and talk about their relationship.

This could happen during their reunion in the summer for Princess Diana's death anniversary. The last time that Prince Harry and Prince William appeared together and showed a united front in public was in July, last year. They were both on hand to unveil their mother's statue at the Sunken Garden in Kensington Palace.

William and Harry at Pippa Middleton wedding
Prince William, Duke of Cambridge and Prince Harry arrive for the wedding ceremony of Pippa Middleton to James Matthews at St Mark's Church on 20 May Arthur Edwards - WPA Pool/Getty Images